Sludge Reduction: Technologies Integrated in the Sludge Handling Units
Content Table
- Chemical and thermo-chemical hydrolysis
- Enzymatic hydrolysis with added enzymes
- Mechanical disintegration
- Microbial predation
- Oxidation with ozone (ozonation)
- Thermal treatment
- Ultrasonic disintegration
- References
- Links
Chemical and thermo-chemical hydrolysis
Acid or alkaline thermal hydrolysis is applied as: (1) pre-treatment prior to anaerobic digestion, to improve sludge biodegradation, reduce digester volume and enhance biogas production; (2) treatment of thickened sludge before dewatering, to reduce solids to be disposed of and increase solid content in dewatered cake. As in the thermal treatment, part of the heat used for the thermo-chemical pre-treatment can be recover for heating the mesophilic or thermophilic anaerobic process.
In the configuration (1), since methanogenic bacteria activity and thus the production of methane is strongly affected by pH, which should be maintained in the optimal range of 6.6–7.6, pH neutralisation after strong acid/alkaline conditions may be needed. However, after sludge hydrolysis pH changes spontaneously and may becomes again suitable for biological processes.
Digestion of sludge pre-treated at temperatures up to 170-175°C resulted in an effective increased methane production, while higher temperatures (>180°C) do not lead to a further increase. Treatment at 100-120°C with acids or alkalis improves dewaterability of thickened sludge (5-6% TS content), leads to a reduction of the sludge mass to be dewatered (inducing solubilisation) and increases the dry content of sludge cake. The water phase separated from dewatering, rich in solubilised organic compounds, N and P, is recirculated to the wastewater handling units.
Enzymatic hydrolysis with added enzymes
Sludge solubilisation by enzymes is more intense the nearer the temperature is to the 50°C, considered optimal for hydrolitic enzyme activity. For this reason, the enzymatic treatment is best applied with anaerobic digestion in mesophilic or thermophilic conditions. Hydrolytical enzymes have the role to improve the hydrolysis step prior to acidogenesis, which is well known to be the rate-limiting step for the anaerobic digestion.
The treatment of sludge with various combinations of commercial protease, lipase, cellulase, hemicellulase, glycosidic enzymes has been tested for the enzymatic treatment + anaerobic digestion, with the aim to improve sludge reduction and enhance biogas production. The hydrolytic enzymes added in anaerobic reactors at lab-scale, demonstrated effectively good results in floc disintegration, significant reductions in EPS, better filterability and higher biogas production. Dewatering properties can be also improved, leading to a reduction of total sludge volume and a minor polymer dosage for dewatering. Although proteases are very effective in reducing sludge solids and in improving settling, when used glycosidic enzymes, they resulted in better solubilisation of the sludge than proteases and lipases.
Some types of cation binding agents can have a role to remove cations as Ca2+, Mg2+ or Fe3+ from flocs, favouring the disruption of floc structure end enhancing the enzymatic activity during methanogenesys.
The installation of a dosage system for the enzymes represents the only investment cost, but the cost of enzyme mixtures has to be considered as the most important and sometimes limiting aspect for their use at full-scale.
Mechanical disintegration
Mechanical disintegration is often proposed as integration in the sludge handling units, which is a preferable configuration rather than the integration in the wastewater handling units. The reason is that it is more economic to operate with high concentrated sludge (thickened), in order to reduce the energy needed for disintegration. It is possible to insert a pre-treatment of sludge before anaerobic digestors or the treatment on a part of the sludge return flow.
The mechanical disintegration, due to the reduction of size and compactness of biological flocs, enhances the contact among bacteria, substrates and enzymes. Therefore the biodegradability of sludge is enhanced with a consequent increase in biogas production in anaerobic digestion.
With a high disintegration level the sludge dewatering performance is enhanced and a higher dry content is obtained respect to the untreated sludge. An increase of the flocculant dosage used for sludge conditioning and dewatering is observed, probably required for the neutralisation of superficial charges of colloidal particles increased in number after the mechanical treatment.
In some experiences, it was observed that foaming problems in anaerobic digesters – caused by filamentous microorganisms – can be reduced by adopting mechanical disintegration as pre-treatment.
Microbial predation
Microbial predation by worms may lead to substantial and cost-effective sludge reduction, but up to now knowledge of the process is not fully complete and therefore the application of predators for sludge reduction at full-scale is still not entirely manageable and predictable. In the proposed configuration for treating excess sludge after production, an additional specialised predation-reactor can be integrated with the sludge handling units, using Tubificidae or Lumbriculidae for sludge reduction by transforming it into worm faeces. In new reactor concepts, the separation of the worms from the excess sludge was obtained by the immobilisation of L. variegatus in a carrier material, which allows the worms to be recovered as a valuable protein-rich product from the waste sludge.
The application of this process is still at lab-scale; in full-scale applications, the need for large areas of the carrier material may affect economic feasibility.
Oxidation with ozone (ozonation)
The advanced oxidation processes applied to sludge reduction and integrated in the sludge handling units consists of the use of ozone, hydroxide peroxide or oxygen at high temperature and high pressure.
The efficiency of both anaerobic and aerobic stabilisation processes can be enhanced by integrating sludge ozonation. The partial solubilisation of sludge favours the hydrolysis of organic matter which is the main limiting factor of digestion processes.
The most widely used configuration is the integration of ozonation with the anaerobic mesophilic digesters with the aim of reducing sludge mass and enhancing methane production to cover the costs of the additional ozonation treatment. In this configuration, ozonation can be applied as: (1) a pre-treatment prior to the digester or (2) a post-treatment, operating on the return flow of digested sludge.
Although sludge filterability is generally deteriorated by ozonation, in some experiences it was observed that dewaterability was significantly enhanced after ozonation + anaerobic digestion, comparable with the dewaterability of untreated sludge. In this case a significantly lower production of dewatered sludge cake with lower water content can be obtained, contributing to savings in sludge disposal costs.
Thermal treatment
The thermal treatment is integrated in the sludge handling units with the aim to (a) reduce sludge production, (b) enhance biogas production in anaerobic digesters and (c) obtain pathogen inactivation, (d) improve sludge dewaterability.
The integration of thermal treatment in the sludge handling units is generally preferred with respect to the integration in the wastewater handling units, due to the higher solid concentration (5-7% TS) in thickened sludge, resulting in a energy saving for heating and in a reducing reactor volume. The energy consumption for sludge heating can be covered by biogas production when thermal treatment is used as a pre-treatment before anaerobic digestion. The thermal treatment applied at full-scale is in general performed at temperatures in the range 160-180°C with short contact time around 0.5-1 h. Higher temperatures seem to limit the improvement of methane production due to the formation of slowly or hardly biodegradabile products.
Thermally hydrolysed sludge has a better dewaterability (resulting in a dry content of sludge cake up to 35%), lower viscosity and appears as a liquid even at solid content around 12%.
Ultrasonic disintegration
Ultrasonic disintegration is integrated in the sludge handling units as a pre-treatment before anaerobic digestion to obtain sludge reduction and an increase in biogas production. The application in the sludge handling units disintegration, is more effective because the higher TS concentrations in treated sludge. In the presence of a higher TS concentration in the bulk liquid, a higher number of cavitation sites occurs and the probability that the solids come into contact with exploding cavitation bubbles increases. However, in practice, it is not be feasible to use too high TS concentrations, because there is a risk of overheating, sonotrode erosion and plant breakdown.
At the energy levels usually applied in practice, sonication causes floc size reduction without damaging cells, because damage/death of bacteria requires too much energy (> 60.000 kJ/kgTSS). However, the increase in the specific surface of sludge flocs favours contact among bacteria, substrates and enzymes, enhancing the overall sludge biodegradability and biogas production, as demonstrated by several full-scale applications as a pre-treatment before anaerobic digestion. The ultrasonic disintegration requires very high energy, but this drawback is offset by the ease of installation, the simple managementa and the compactness.
Reducing the specific energy applied in the pre-treatment before anaerobic digestion to more sustainable values, the benefit related to the VS mass reduction is reduced, but the increase in biogas production is always observed.
References
Links
Reduction Sludge Production in Wastewater treatment Plants
Sludge Reduction: Technologies integrated in the Wastewater Handling Units
